When a child with hyperphenylalaninemia (HPA) due to a defect in BH4 synthesis was treated with subcutaneous injections of BH4, only about 10% of the oral dose was required to get a comparable neurological and biochemical response. A variant form of HPA due to a defect of BH4 has been demonstrated in which the defect is restricted to the periphery. In order to evaluate the need for BH4 replacement therapy in HPA due to BH4 deficiency, pterin and biogenic amine metabolite levels in CSF must be determined.